Caption: Red blood cell. Coloured scanning electron micrograph (SEM) of a human red blood cell (erythrocyte). Red blood cells are biconcave, giving them a large surface area for gas exchange, and highly elastic, enabling them to pass through narrow capillary vessels. The main function of red blood cells is to distribute oxygen to body tissues and to carry waste carbon dioxide back to the lungs. magnification: x10000 when printed at 10 centimetres wide.